1. Technical Field
The present invention relates to a magnetic resonance imaging apparatus.
2. Description of Related Art
A magnetic resonance imaging apparatus is an apparatus that applies a radio-frequency magnetic field onto a subject placed in a static magnetic field, detects a magnetic resonance signal generated from the subject due to the application of the radio-frequency magnetic field, thereby creating an image. Such magnetic resonance imaging apparatus includes a gradient coil that adds spatial positional information to a magnetic resonance signal by applying a gradient magnetic field onto the subject.
The gradient coil is repeatedly supplied with a pulse current while imaging, thereby generating heat to a large extent. Especially in recent years, a speedup in switching of a gradient magnetic field and an enhancement of the strength of a gradient magnetic field are strongly required along with the speedup in an imaging technology, so that heat generation by a gradient coil becomes more marked.
Heat generation by a gradient magnetic field has a possibility of giving influence on the quality of a taken image, or giving a pain to a subject to be imaged. For this reason, for example, a technology of cooling a gradient coil while imaging by circulating a coolant through cooling pipes provided inside the gradient coil has been proposed (for example, see JP-A 2006-311957 (KOKAI)).
FIG. 11 is a schematic diagram for explaining an example of a conventional technology using cooling pipes. FIG. 11 depicts an Active Shield Gradient Coil (ASGC) that is generally used as a gradient coil. In FIG. 11, the upper surface depicts an outer circumferential surface of the ASGC, and the lower surface depicts an inner circumferential surface of the ASGC. An imaging area in which a subject is to be placed is formed on an inner side of the ASGC.
As shown in FIG. 11, the ASGC includes a main coil 1 and a shield coil 2 each of which is formed in a drum shape. Moreover, through-holes forming openings at both ends of a gradient coil 20 are formed as shim-tray insert guides 3, for example, between the main coil 1 and the shield coil 2. In each of the shim-tray insert guides 3, inserted is a shim tray 4 that accommodates therein iron shims 5 that are for correcting ununiformity of a static magnetic field in the imaging area in which the subject is placed. Furthermore, on the inner side of the main coil 1, provided is a Radio Frequency (RF) shield 7 for shielding a radio-frequency magnetic field generated by an RE coil arranged on the inner side of the ASGC.
According to such ASGC, for example, a main-coil side cooling pipe 6a formed in a spiral fashion is provided between the main coil 1 and the shim-tray insert guides 3. Moreover, a shield-coil side cooling pipe 6b formed in a spiral fashion is provided between the shield coil 2 and the shim-tray insert guides 3. The ASGC is cooled by circulating a coolant, such as water, through the main-coil side cooling pipe 6a and the shield-coil side cooling pipe 6b. 
However, recently, magnetic resonance imaging apparatuses tend to have a large diameter of a bore that forms an imaging area. As the diameter of the bore getting larger, the diameter of a gradient coil also becomes larger, consequently, electricity supplied to the gradient coil becomes larger. As a result, heat generation by the gradient coil is increased to much greater extent, correspondingly, a temperature rise in the imaging area on the inner side of the gradient coil becomes more marked.